1. Field of the Invention
This invention relates to heat developable light-sensitive materials and particularly to heat developable light-sensitive materials which contain certain kinds of esters as a reducing agent, and which have reduced thermal fog at the background, increased whiteness and high stability to normal room illumination due to presence of these esters.
2. Description of the Prior Art
Photography using silver halide has been much more universally carried out till now, as compared with electrophotography, diazo photography and the like, because the former has superior photographic characteristics such as sensitivity, gradation, etc., than the latter. However, silver halide photography requires much time and labor, since silver halide light-sensitive materials employed therein must be subjected to several processings using solutions comprising an image-exposure process; a developing process with a developer; and processes for preventing the developed-image from changing color or deteriorating under indoor illumination or protecting the non-developed white areas (hereinafter called "background") from blackening; that is, a stopping, a fixation, a washing and rinsing, a stabilizing and other similar processes. In addition, chemical agents used in this method are dangerous to the human body, and the processing room and the workers' hands and clothes are often stained by these agents. Moreover, pollution difficulties arise by discharging processing solutions into rivers. Therefore, it has been strongly desired to improve silver halide-containing light-sensitive materials with high sensitivity so as to deal with them in a dry condition instead of processing with solutions, so as to maintain the developed-images stable, and so as to reduce the color change of the background under indoor illumination.
In order to solve these problems, many efforts have been made. For example, the addition of 3-pyrazolidone system as a developing agent into a silver halide emulsion has made it possible to develop by application of heat as disclosed in German Pat. application OLS Nos. 1,123,203; 1,174,157 and so on. In addition, the coexistence of water-generating substances with the above-mentioned developing agent in the course of heating can promote the developing reaction as disclosed in German Pat. application OLS No. 1,175,075. Moreover, the effect of a fixing agent for silver halide when added together with water-generating substances to the above-mentioned developing agent is described in German Pat. application OLS No. 1,003,578. However, the above techniques do not make it possible to completely stabilize the silver halide itself, which remains in the finished light-sensitive material processed under dry conditions, to light. Namely, fixation under dry conditions is not described in the above three patent specifications, and as would be expected, the coexistence of a developing agent (reducing agent) with a fixing agent, which is described in German Pat. application OLS No. 1,175,075, will cause undesirable reactions on storage, and they will, therefore, render this technique industrially impractical. These have been experimentally confirmed.
In the art of light-sensitive materials which provide photographic images using photographic treatments under dry conditions, the most successful materials have been heat developable light-sensitive materials which were disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075; and wherein the composition containing as essential components a silver salt of an organic acid, a small amount of a silver halide and a reducing agent was employed. In the above light-sensitive system, silver halides remaining in the light-sensitive materials after developing processing were allowed to remain even though they changed color, without stabilizing them to light. Nevertheless, the above light-sensitive system as well as those light-sensitive materials containing residual silver halide which received a stabilization treatment for light produced satisfactory results. This was because only a small amount of silver halide was used therein, a large portion of the silver component was present in the form of white or pale yellow organic silver salts which were so stable to light that they hardly blackened on exposure, and a slight coloration resulting from the decomposition of a small amount of residual siler halide by application of light is hardly perceived by the human eye. The above light-sensitive system is stable at ordinary temperature, but when heated up to about 80.degree. C, and preferably 100.degree. C, after image-exposure, an image is formed because an organic silver salt as an oxidizing agent and a reducing agent incorporated in a light-sensitive layer undergo a redox reaction in the presence of a catalytic amount of exposed-silver halide present in the vicinity thereof, to result in the liberation of silver which blackens quickly the exposed area of the light-sensitive layer and causes a contrast to the unexposed area thereof (background).
The present invention relates to an improvement in the above-described heat developable light-sensitive materials, particularly the reducing agents incorporated therein, which is carried out with the intention of producing satisfactory results as described hereinafter. In order to use reducing agents as an effective constituent of heat developable light-sensitive materials, they must be able to reduce organic silver salts when heated in presence of the catalyst of the exposed silver halide, that is, to have a heat developable property. Reducing agents which have already been proposed in the patent art and the like include substituted phenols, substituted or unsubstituted bisphenols, substituted or unsubstituted mono- or bis naphthols, di- or poly-hydroxybenzenes, di- or poly-hydroxynaphthalenes, hydroquinone monoethers, ascorbic acid or the derivatives thereof, 3-pyrazolidones, pyrazoline-5-ones, reducing saccharides, p-phenylendiamine or the derivatives thereof, reductones, kojic acid, hinokitiol, hydroxylamines, hydrooxaminic acids, sulfohydrooxaminic acids, hydrazides, indane-1,3-diones, p-oxyphenylglycine and so on. A number of patents are concerned with these reducing agents, as disclosed, for example, in U.S. Pat. Nos. 3,152,904; 3,457,075; 3,531,286; 3,615,533; 3,679,426; 3,672,904; 3,751,252; 3,751,255; 3,782,949; 3,770,448 and 3,773,512; German Pat. application OLS No. 2,031,748 and Belgian Pat. No. 786,086.
However, these reducing agents undesirably blacken the unexposed area during the heat developing process, namely they produce much thermal fog. Even if heat-developed for a comparatively short time, the light-sensitive materials containing such reducing agents also have a light yellow-brown stain at the background. Moreover, the background of these processed light-sensitive materials becomes yellow-brown due to scattered light in a room.
The solution to the above-described problems has been an important subject in the art of heat developable light-sensitive materials.